Opposites attract: Stable and unstable materials
couple for high performance

n ear-splitting, bone-jarring ride in a small, propeller-driven airplane
illustrates the need for better materials that deaden vibration, says
Professor Walt
Drugan. “You’d very much like it to be full of high-damping
materials,” he says. But materials with such extreme properties,
including high stiffness and high damping, don’t yet exist.

With a four-year, $800,000 grant from the National
Science Foundation’s Engineering Directorate-Division of Civil
and Mechanical Systems, Drugan, Wisconsin Distinguished Professor Rod
Lakes, Assistant Professor Rob
Carpick, and Materials Science & Engineering Professor Reid
Cooper are studying how to couple stable and unstable materials
to yield new high-performance materials. In the structural stiffness
sense, says Drugan, if you push on a stable material, it will push back,
like a spring. An unstable material will push away or revert to a form
in which positive, or stable, stiffness governs.

And while the idea of composite materials has
been around literally forever—for example, early bricks of mud
and straw—combining two opposite-reacting materials in just the
right quantities and geometries to get super-high performance is very
new, he says.

Generally, when people do the mathematics of
materials, they assume everything is stable, says Lakes. “What
we’ve found is you don’t necessarily have to assume that,
because you can have an unstable part that’s stabilized by other
parts that are in fact stable,” he says.

In a 2002 paper in the Journal of the Mechanics
and Physics of Solids, he and Drugan showed theoretically that combining
stable and unstable materials in a certain way would produce materials
with “super” properties such as high damping or stiffness.

In addition to an aircraft or spacecraft, there
are many situations in which such materials are desirable, says Lakes,
including quieter cars, more stable bridges and tall buildings, or more
efficient electronics.

The group, which also includes graduate students
Amelia Berta, Yun-Che Wang, Pat Frascone, Tim Jaglinski and Sara Blair,
began the project last fall and is focusing not only on the general
theory, but also on developing specific material systems. Carpick is
applying the work to carbon nanotubes, while Cooper is investigating
materials that easily undergo crystal transformations.